A Conceptual Model Perspective on the Coupling of El Niño–Southern Oscillation and Global Mean Surface Temperature

Abstract

El Niño-Southern Oscillation (ENSO), the most energetic source of inter-annual climate variability, affects not only the regional climate in its tropical Pacific origin but also the global climate. During El Niño events, elevated sea surface temperatures (SSTs) in the eastern equatorial Pacific release excess heat into the atmosphere, leading to a delayed warming of the global mean surface temperature (GMST). In the context of a warming climate with increasing GMST, ENSO is expected to change, but the exact nature of this change remains uncertain. To capture the bidirectional relationship of ENSO and GMST, we developed a simple full-field model for GMST and coupled it to an existing conceptual model for ENSO through SSTs and radiative-convective equilibrium (RCE) temperature. Our model shows a reduced frequency of El Niño events but stronger individual occurrences in a warmer climate. The response of GMST to ENSO remains approximately constant at around 0.12◦C per degree of Niño 3.4 index with a lag of about four months. Contrary to projections from most general circulation models (GCM) that simulate a weakened zonal SST gradient and trade winds in a warming climate, our results indicate an increasing zonal SST gradient and stronger westerlies sustained by cool eastern Pacific subsurface temperatures; findings that are generally supported by reanalysis data from recent decades. By leveraging the conceptual nature of our ENSO-GMST model, we explore the physical mechanisms behind this behavior, offering an intra-tropical Pacific view on these mean state changes.